Expandable Graphic Equaliser


Posted on Feb 5, 2014

The project described in this article is a constant Q, fully expandable graphic equaliser. Where most conventional  graphic EQ circuits have a Q that is dependent on the setting of the pot, this one maintains the same Q at all settings. This is achieved by using MFB (Multiple Feedback Bandpass) filters, instead of the more common gyrator  tune


Expandable Graphic Equaliser
Click here to download the full size of the above Circuit.

d circuit. As always, there are pros and cons for the approach described here. Phase shifts tend to be a little more radical, and the passband has more ripple than a conventional circuit, but only where a number of sliders are set to boost or cut. On the positive side, specific frequencies are dealt with specifically regardless of the level, and not with a variable Q. The constant Q circuit makes room equalisation and feedback reduction far better behaved. So much better in fact, that a boost or cut of 3dB or less may provide the required effect, where a variable Q equaliser may need considerably more, and will affect the adjacent frequencies to a far greater degree. The filters used are the same as in the Instrument Graphic Equaliser, and are multiple feedback bandpass types. An example of this filter is shown in Figure 1, and more details are available from the project page for the MFB filter. Depending on the configuration you ultimately decide upon, you will need between 10 and 30 of these filters -per channel for stereo! This circuit is reproduced from the original article for convenience the actual filter circuits used are slightly different, and are shown in Figure 3. Building 60 of these may sound like an awful chore, which is perfectly reasonable, since it will be just that. With this knowledge at hand, this may go some way to help you make some Now you have to decide on the frequency resolution. 1/3 octave would...




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